Friday, April 25, 2014

This forecast is solely for the severe weather on Monday, April 28th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has outlined a rather large area of potential severe weather to kick off the workweek on Monday, April 28th. This is a continuation of the severe weather events on Saturday and Sunday, as a strong upper level low continues to ravage the country. When the SPC issues these sorts of long range outlooks, they highlight areas with a 30% chance or more of observing severe weather within 25 miles of any given point. This means that any of the areas highlighted in Arkansas, Louisiana, Mississippi, Alabama, Tennessee, Kentucky, Indiana, Illinois and Missouri may see some formidable severe weather, with a lesser-but-still-present severe weather threat surrounding this circular shape.

A look at the projected 500mb isobars and wind speeds for late Monday evening continues to exemplify why this event will be a multi-day, potentially significant severe weather outbreak. We see a long strand of anomalously high wind speeds extending from Oregon, rounding the closed low and punching into the area we're watching for Monday's severe weather risk around Missouri and Illinois. Although the closed low does not have a negative tilt that could then intensity the situation further, it does look like this piece of the Pacific jet stream will be what keeps the severe weather going, even though the closed low won't be doing much, other than continuing to pull moisture up from the Gulf of Mexico.

A look at 700mb winds, just a few thousand feet off the ground for the same time as the 500mb image shows strong lower-level winds over the area we're watching for the severe weather threat Monday evening. Considering the streak of 40 to 60 knot winds in the Midwest and southern Plains will only continue to intensity during Monday evening, as it is the nocturnal Lower Level Jet stream (LLJ), the severe weather threat will likely continue as well, at least through Monday evening.

A look at projected dewpoints and the Lifted Index for Monday evening really emphasizes the moisture this system has to work with. We see the projected dewpoints in shaded colors, and the dark blues represent the highest dewpoints, and thus some pretty muggy air. This all bodes well for severe weather potential, which is confirmed by the very low Lifted Index values, some as low as -9. The Lifted Index, also called the LI, is a measure of buoyancy of the air- in other words, when the LI is negative, air tends to rise and thus create thunderstorms. When the LI is positive, it indicates a stable atmosphere, suppressing thunderstorms. Seeing numbers close to double-digit negatives tells me this closed low will be keeping the severe weather fireworks going well into the workweek.

To summarize:
• A potentially significant severe weather event is possible on Monday.
• Those in the outlined area may want to review severe weather preparations and watch for further updates on this situation.

This forecast is solely for the severe weather on Sunday, April 27th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has issued a severe weather outlook for Sunday, April 27th, indicating the continuance of Saturday's severe weather. In the graphical representation of the severe weather risk above, we see there is a large 'slight risk' of severe weather covering Nebraska, Kansas, Oklahoma, Texas, Louisiana, Mississippi, Tennessee, Kentucky, Arkansas, Indiana, Iowa and Missouri. We also see a much more alarming 'Moderate risk' of severe weather, covering extreme northeast Texas, southwest Arkansas, northwest Louisiana and southeast Oklahoma. This moderate risk is particularly alarming, as it appears to be only the 9th time in Storm Prediction Center history that a moderate risk has been issued two days away from the predicted event.

The image above shows the Storm Prediction Center's graphical representation of the percentage chance of severe weather within 25 miles of any given point. We see the 'slight risk' area as defined by the 15% chance swath, with an enhanced severe weather chance in the 30% red region. But it's the 'moderate risk' area that wins the contest, clocking in at a 45% chance of severe weather within 25 miles of any point. Putting that in simpler terms, you could flip a coin, and there's just about a 50/50 chance of severe weather within 25 miles of that purple area. The black hatched area signifies a 10% or greater chance of significant severe weather (i.e. very large hail, very strong winds or strong tornadoes).

The image above shows isobars (contour lines) and wind speeds (colored) at the 500mb level of the atmosphere on Sunday evening. Here, we see our strong trough that we went over in the Saturday Outlook post has now become a closed low, as seen by the closed isobars in Kansas and Nebraska. This indicates that the storm has passed maturity, and is now on the path to weakening. Despite this, a screaming mid-level Pacific jet stream, clocking in at nearly 100 knots, will be feeding this severe weather threat for days to come. We see the closed low wrapping those strong mid-level winds into the warm sector, enhanced right over that meeting point of Texas, Arkansas and Oklahoma, and right over the Moderate Risk outline. These strong winds indicate the enhanced risk of severe weather.

If we look at wind speeds at the 700mb level, just a few thousand feet off the ground, we see wind speeds of close to 60 knots enhanced over our severe weather area, and especially over that Moderate Risk outline of Texas, Arkansas and Oklahoma. Seeing as this 700mb wind component is also known as the lower level jet stream (LLJ), and is a nocturnal jet stream, we can expect these winds to sustain themselves as Sunday wears on, thus continuing the severe weather threat. These high wind speeds combined with the high mid-level winds we just went over make me more and more concerned for Sunday.

According to model guidance, there is every right to be concerned. The image above shows the forecasted supercell composite for Sunday night. In other words, the higher colors on the legend indicate the higher likelihood for a supercell, which is defined as a rotating thunderstorm. Seeing as the supercell composite is enhanced right over that moderate risk area again, it's clear that this focal point of Louisiana, Arkansas, Texas and Oklahoma will be the bullseye for Sunday's severe weather threat.

Interestingly enough, the forecasted significant tornado composite, which is an index that can help determine the likelihood of tornadoes in a given area, goes off the scale right over the Moderate Risk area, but in my opinion, it's best to wait another day or so before jumping on this train of violent tornadoes (though I will agree that strong tornadoes are possible here).

To summarize:
• A potentially significant severe weather event is expected to occur Sunday.
• A Moderate Risk has been issued for Oklahoma, Texas, Louisiana, and Arkansas.
• Those in the Moderate Risk area may want to prepare for potentially strong tornadoes on this day.

This forecast is solely for the severe weather on Saturday, April 26th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center is highlighting an enhanced severe weather risk over the Southern Plains on Saturday, April 26th. The image above shows a graphical representation of the SPC's thoughts for Saturday's severe threat, with the percentages showing the likelihood of severe weather within 25 miles of any given point. In this graphic, we see that the states of Nebraska, South Dakota, Iowa, Kansas, Oklahoma, Missouri and Texas are in the general 'slight risk' categorical severe weather outlook, indicated by the yellow 15% shading, while south-central Kansas, central Oklahoma and north Texas are in a 30% swath of enhanced severe weather chances. We also see the black hatched denotation, meaning areas within that have a 10% or greater chance of seeing significant severe weather within 25 miles of any point (think extremely large hail, very high winds, and strong tornadoes instead of "tamer" severe weather). Let's go over how this will evolve.

The projected surface analysis on Saturday evening from the Weather Prediction Center shows our strong storm located in northeast Colorado, with a warm front extending through the Central Plains and snaking south into Arkansas and Louisiana. We then identify our cold front way back west in Colorado and New Mexico, but we also see an orange boundary in western Texas, Oklahoma and Kansas. What is that boundary? That is a dryline. In the same sense that warm and cold frontal boundaries separate warm and cold air, a dryline boundary separates dry and moist air, sometimes creating a substantial temperature gradient in the process. Drylines are the source of numerous storms in the Plains, and look to be the source for Saturday's severe weather, as well.

On Saturday evening, we will begin seeing a strong trough (storm system) in the Southwest begin to attain a negative tilt. A negative tilt is visually seen as the isobars trying to 'push' towards the south-east direction, indicating the storm system has reached maturity, and we see that phenomenon occurring in the 500mb vorticity forecast map above. This negative tilt looks to be attained in the late evening hours of Saturday, which brings up some timing issues for this severe weather event on Saturday, which we will discuss more next. The general thing to take away here, though, is that this will be a very strong storm system surging eastward to kick off this multi-day significant severe weather outbreak.

There does look to be some timing issues on Saturday, not just with respect to how fast the storm can acquire a negative tilt, but also with the presence of a capping inversion Saturday evening. Shown above is the Saturday evening forecast for instability, marked in j/kg by contour lines, and stability, shown by the presence of blue shading; the darker the blue shading, the higher the stability. Note that instability means air can rise because the air at the surface is warmer than the air above the surface, and stability means air cannot rise, as the air above the surface is warmer than the air at the surface. If we recall that air can only rise if the surrounding air is colder than the surface, the presence of stability means thunderstorms cannot form. In this forecast image, we see that there is a lot of projected instability over the Southern Plains, over 3000 j/kg in some places, which is a very high amount of instability. In some spots, we see white, which means no capping inversion, but across the remainder of the SPC-outlined severe weather risk, we see a significant capping inversion in place, as the dark shading shows. I'm worried that this cap will be too strong to break through, and the trough will be too slow attaining that negative tilt and moving closer to the Southern Plains leading to a bust in the forecast.

Let's hypothesize for a moment that storms do form on Saturday. A weather model named the WRF-ARW can predict the maximum updraft helicity for the entire 48 hours it forecasts for. In this instance, it means we can see how strong the storms that form rotate and spin from the morning of April 25th to the morning of April 27th, encompassing Saturday's severe weather in the process. We see on this image that the WRF-ARW model projects multiple tornadic storms to develop across the Central and Southern Plains, with two notably strong supercells violently rotating from southwest Oklahoma to the central portion of that state, and another from western to eastern Kansas. So while the environment may not be too favorable right now, any storms that do form and can sustain themselves look to be rather significant.

To summarize:
• A potentially significant severe weather event may occur on Saturday.
• The Central and Southern Plains would be affected.
• Model guidance is forecasting an inhospitable environment for storms, which may greatly hamper the severe weather threat.
• Any storms that do form have the potential to be significant and potentially tornadic.

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